The present invention generally relates to rotational acceleration sensors (i.e., angular accelerometers) and, more particularly, to a microfabricated angular accelerometer.
Angular accelerometers are employed to measure the second derivative of angular rotation with respect to time. In some machine controlled applications, a measurement of angular acceleration is often needed as a direct input to a control system. For example, in order to prevent against disturbances from external angular acceleration, disk drive read/write heads generally require a sensor for sensing angular acceleration so that the control system associated therewith may compensate for the severe shock and/or vibrations that may have caused the angular acceleration to occur.
One approach to determining angular acceleration employs an angular velocity sensor to sense angular velocity, and differentiates the sensed angular velocity to determine the angular acceleration. The design for an angular velocity sensor is usually complex, and angular velocity sensors are generally expensive to produce. In addition, the acceleration measuring devices employing an angular velocity sensor typically require a differentiator which adds to the complexity and overall cost of the device.
Another approach for determining angular acceleration uses a combination of two linear accelerometers mounted to a rigid body for sensing linear acceleration along two respective perpendicular axes. Generally, linear accelerometers employ a mass suspended from a frame by multiple beams. The mass, beams, and frame act as a spring-mass system, such that the displacement of the mass is proportional to the acceleration applied to the frame. The signal extracted from two linear accelerometers can be used to extract angular acceleration information. Linear accelerometers are readily available and easy to use; however, in order to measure angular acceleration while rejecting linear acceleration, the scale factor, i.e., sensitivity or gain, of the two sensors generally must be matched.
A further approach for an angular accelerometer is disclosed in U.S. Pat. No. 5,251,484, entitled xe2x80x9cROTATIONAL ACCELEROMETER,xe2x80x9d which employs a circular hub centrally supported on a substrate and connected to radially disposed thin film spoke electrodes that flex in response to angular acceleration. Rotational acceleration measurement is achieved by using a differential, parallel plate capacitive pick-off scheme in which the flexible spoke electrodes at the periphery of the fixed disk rotate between fixed reference electrodes so that an off-center position of moving electrodes results in a measured differential voltage from which the disk motion is determined. The sensing capability for such an accelerometer is generally limited to the amount of movement of the flexible spoke electrodes. This cantilevered design with rotary electrodes generally requires high structural matching to assure predictable gain, phase, and linearity response. The linear and cross-axes sensitivity (gain) is highly dependent on the structural matching. Additionally, separate input and output contacts for each capacitive plate add to the overall complexity and cost of the accelerometer.
The angular accelerometer disclosed in related U.S. application Ser. No. 09/410,712, filed on Oct. 1, 1999, to the Assignee of the present application, discloses an angular accelerometer having a rotational inertial mass suspended over a cavity and connected via tethers at the outer periphery to a substrate. The angular accelerometer disclosed in the aforementioned application achieves enhanced sensitivity over prior known approaches. In some environments, prior known accelerometers may be sensitive to stresses induced by fabrication processing, packaging, handling, and structural asymmetries, and may be susceptible to damage.
Accordingly, conventional accelerometers often suffer from various drawbacks including errors introduced by rotational acceleration orthogonal to the sensing axis and errors introduced by linear acceleration, as well as being susceptible to poor sensitivity and damage. It is therefore desirable to provide for a low cost, easy to make and use, enhanced sensitivity angular accelerometer that eliminates or reduces the drawbacks of prior known accelerometers.
In accordance with the teachings of the present invention, an angular accelerometer is provided having a substrate, a fixed electrode supported on the substrate and including a first plurality of fixed capacitive plates, and a rotational inertia mass substantially suspended over a cavity and including a plurality of movable capacitive plates arranged to provide a capacitive coupling with the first plurality of fixed capacitive plates. The rotational inertia mass is configured as a substantially annular ring that is rotationally movable relative to the fixed electrode. A fixed central member is attached to the substrate and located substantially at the center of the rotational inertia mass ring. A plurality of support arms extend between the rotational inertia mass and the central member and allow the rotational inertia mass to be biased relative to the fixed electrode during rotational movement of the rotational inertia mass. The angular accelerometer also includes an input electrically coupled to one of the fixed electrodes and the rotational inertia mass for receiving an input signal, and an output coupled to the other of the fixed electrodes and the rotational inertia mass for providing an output signal which varies as a function of the capacitive coupling and is indicative of angular acceleration.
By connecting the rotational inertia mass ring to the fixed central member via the plurality of support arms, the angular accelerometer is less sensitive to stresses induced by fabrication processing, packaging, handling and structural asymmetries. The realization of high gain achieved with the accelerometer enhances immunity to electromagnetic interference (EMI) signals and environmental conditions, such as humidity and temperature. In addition, the angular accelerometer provides high gain for angular accelerations about the sensing axis, while minimizing linear and cross-axis sensitivities.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.